Because of the better coefficient of performance of heat pumps as compared to electrical resistive heating elements, there has been a movement in recent years to purchase electric resistance hot water heaters and then connect the water heater tank to a heat pump so that the heating was actually performed by the heat pump rather than the electric resistance elements in the water heater itself. This produced a good coefficient of performance while at the same time retained the electric resistance heating elements as a standby. One such arrangement is illustrated in U.S. Pat. No. 4,740,673, issued May 26, 1988, entitled "Dual Control Thermostat Circuit" that allows the conventional upper and lower thermostats to control the operation of the external heat pump.
One of the problems associated with heat pump heaters for water heaters of the type shown in U.S. Pat. No. 4,740,673 is that no provision was made to automatically switch to the resistance heaters when the inherent low instantaneous heating capacity of the heat pump was exceeded after a large consumption of hot water. The result is frequent hot water outages and a long waiting period until the entire tank recovered to a useful water temperature. This problem was caused by the fact that the heat pump typically heats the tank from the bottom up and may require several hours before the temperature at the top of the tank reaches a useful level. In a conventional two resistance heating element water heater, this problem is solved since the upper thermostat and resistance element gives priority to heating the top of the tank before power can be applied to the bottom heating element. This tank top priority heating gives the effect of fast recovery since a small amount of hot water at the top of the tank is quickly available before the full tank is heated.